1. Field of the Invention
The present invention relates generally to airborne earth penetrating radar, and more specifically to discriminating between underground objects and surface clutter objects by improving the signal-to-clutter ratio.
2. Description of the Prior Art
The detection, discrimination and identification of buried or subsurface objects is of interest for such uses as environmental range remediation, and the discernment of buried military facilities, unexploded ordinances, tunnels, and toxic wastes. The existence of such buried objects may be detected by the use of an earth penetrating radar ("EPR"), where a transmitted radiant signal is propagated into the ground. If an object is located underground, it will scatter the transmitted signal and reflect a return signal back to the radar for processing. A statistical decision process is employed to indicate the presence of an object based on the returned radar signal.
When an electromagnetic wave of radian frequency .omega..sub.o (where frequency f.sub.o =.omega..sub.o /2.pi.) propagates into the ground, the wave experiences both attenuation and phase changes which generally depend on .omega..sub.o and the relative permittivity or dielectric constant (.epsilon..sub.r), and conductivity (.sigma.) of the earth. The attenuation of electromagnetic radiation in most types of soil rises with the frequency and with the water content of the soil. Frequency-dependent attenuation in the ground sets a limiting value of the effective bandwidth, which thereby limits the information content of a radar signal at a given noise or clutter level. Where the frequency is high enough, the attenuation is frequency independent while the phase change is proportional to .omega..sub.o. A radar signal whose spectrum of frequencies is high enough to meet these conditions will not be dispersive. When a transmitted signal S.sub.T (t) is propagated into the ground from a surface platform, then the return signal S.sub.R (t) is defined by EQU S.sub.R (t)=AS.sub.T (t-t.sub.d)
where A is a time-independent parameter that depends on the radar signal waveform, ground constants, target cross section and range; t.sub.d is the round trip delay between the radar and the target. The contribution to the return signal from noise may be represented by adding the function n(t) on the right side of the equation.
Existing earth penetrating radars are generally used for detecting objects buried at shallow depths of no more than one or two meters underground. These types of radar are often of the ground coupling type having a surface platform where the radar equipment is used at or near the ground surface. For shallow depths, earth penetrating radars of the ground coupling type operate in a frequency regime which is essentially dispersion-free. The close coupling between the radar and the ground has the effect of eliminating surface clutter, and of maximizing the electromagnetic coupling of the radar signal into the ground. This reactive coupling tends to focus the radar signal into the ground and reduce stray horizontal radiation. The return signal is a time-delayed replica of the transmitted signal S.sub.T (t.sub.). The mathematical structure of the return signal from the buried object is identical to the mathematical structure of the transmitted signal. However, this conventional type of radar detection generally will not be applicable for detecting more deeply buried objects where the environment is of a dispersive nature.
In addition, for a radar system utilizing an airborne platform, as shown in FIG. 1, the return signal would consist of not only the return from the buried object, but also the return signals from discrete objects situated along on the earth's surface. The return signal from the discrete objects is referred to as surface clutter, which provides a serious limitation on the ability of airborne radar to detect underground objects using conventional radar signal processing techniques. Returns from surface objects will generally have larger amplitudes than returns from buried objects, and returns from surface objects will also generally have the same signal shape. The signal-to-clutter ratio is small in this situation. Improved detection of objects against such unwanted returns from surface clutter may be achieved by improving the signal-to-clutter ratio of the radar system.
Various prior patents generally illustrating of the state of radar art, including the detection of underground objects, are listed below.
U.S. Pat. No. 5,247,302 issued to Michael S. Hughes on Sep. 21, 1993 describes an entropy-based signal receiver in which changes in received signals as a function of time are scrutinized. Hughes uses changes in the signal level or the signal structure to measure changes in the signal as a function of time or origin. However, Hughes does not measure whether the received signal is a delayed and attenuated signal version of the transmitted signal. Hughes posits that an entropy-based system improves detection of a signal scattered by inhomogeneities in a wave-propagating medium.
U.S. Pat. No. 5,160,931 issued to William M. Brown on Nov. 3, 1992 describes a synthetic aperture radar technique for detecting subsurface objects by cancelling surface echo returns. The radar platform moves perpendicular to a line through a pair of transducers. Both transducers receive return reflection signals. Subsurface objects are detected based on a determination of the complex phase factor in the absence of non-surface objects. The determined complex phase factor is used in cancelling the ground clutter.
U.S. Pat. No. 5,132,691 issued to Lars Hauschultz on Jul. 21, 1992 describes a radar system for recognizing useful signals superimposed with noise signals. A moving time window is used to recognize a useful signal if the duration of the signal exceeds a predetermined amplitude is larger than a predetermined minimum duration which is less than the duration of the time window.
U.S. Pat. No. 4,937,580 issued to Robert H. Wills on Jun. 26, 1990 describes a geophysical radar system producing a high resolution version of a received signal by cross-correlating the received signal.
U.S. Pat. No. 4,905,008 issued to Akio Kawano et al. on Feb. 27, 1990 describes an impulse radar system for detecting the presence of underground objects such as gas pipes. The system transmits a damped sinusoidal periodic wave. A standard time dependent amplification is used to amplify returns from shallow objects close to the surface, and deeper object returns are amplified by a larger amount to compensate for the attenuation due to travel through the earth. A constant amplitude is presented as the depth of the object increases.
U.S. Pat. No. 4,896,116 issued to Yuji Nagashima et al. on Jan. 23, 1990 describes a pulse radar system for distinguishing the echo wave of an underground object from a plurality of echo waves from surrounding objects. The returned signal is divided into portions for every time interval, and the divided signal portion is converted into a corresponding frequency region to obtain a spectral distribution. Spurious echo waves are eliminated based on the frequency region data obtained from the spectral distribution.
U.S. Pat. No. 4,866,446 issued to Hans O. Hellsten on Sep. 12, 1989 describes a coherent all radio band sensing system which can be used to map ice strata and other topographic and geologic structures. The system utilizes the concept of synthetic aperture radar (SAR).
U.S. Pat. No. 4,812,850 issued to David J. Gunton et al. on Mar. 14, 1989 describes a system for combining waveforms to suppress noise and clutter. A correlation operation is used in arriving at two product waveforms which are combined to produce data indicative of the presence of buried objects.
None of the above noted patents, taken either singly or in combination, disclose the arrangement of features in the instant invention for detecting underground objects from an airborne radar platform as disclosed in the present application.